Fix spec violations in burn_charts_spec

What does this MR do and why?

In regards of epic &5102, this MR fixes the following spec violations in ee/spec/frontend/burndown_chart/components/burn_charts_spec.js #391066 (closed):

• spying on methods

How to set up and validate locally

Run tests:

yarn jest ee/spec/frontend/burndown_chart/components/burn_charts_spec.js

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #391066 (closed)